Roll Flutter Engine from a8d861b53db1 to 6e3a04874be5 (1 revision) (#140244)

https://github.com/flutter/engine/compare/a8d861b53db1...6e3a04874be5

2023-12-15 15619084+vashworth@users.noreply.github.com Run tests on macOS 13 exclusively (flutter/engine#49099)

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/flutter-engine-flutter-autoroll
Please CC jsimmons@google.com,rmistry@google.com,zra@google.com on the revert to ensure that a human
is aware of the problem.

To file a bug in Flutter: https://github.com/flutter/flutter/issues/new/choose

To report a problem with the AutoRoller itself, please file a bug:
https://issues.skia.org/issues/new?component=1389291&template=1850622

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

bin/internal/engine.version

-a8d861b53db1aeb84889da5acf2af6de0c8931ef
+6e3a04874be56eadd4bc93c1f9de80695a3f3f61